1. Field of the Invention
The present invention relates to a semiconductor integrated circuit.
2. Related Art
In recent years, a semiconductor integrated circuit has been developed for the use of a video signal or an audio signal and has been in practical use.
The semiconductor integrated circuit for the video signal generally has a charge pump circuit that outputs a voltage by using charge and discharge of the capacitor, a video signal processing circuit block that is applied with the output voltage of the charge pump circuit and a power source voltage to be driven and processes the video signal, and an output terminal that outputs the output voltage of the charge pump circuit to other capacitor.
The semiconductor integrated circuit for the audio signal has an audio signal processing circuit block is applied with the power source voltage and the ground potential to be driven, and processes the audio signal.
Conventionally, the video signal processing circuit block and the audio signal processing circuit block are normally mounted on the separate semiconductor integrated circuits, respectively.
A product having the both functions of the video processing and the audio processing is mounted with a semiconductor integrated circuit for the video signal and one for the audio signal. In this product, if a video signal processing block only is stopped, for example, due to reduction of current consumption, from the state that the video signal processing circuit block and the audio signal processing circuit block are operating simultaneously, the charge pump circuit is turned off. When the charge pump circuit is turned off, its output terminal is made into a High impedance condition. If the output terminal becomes the High impedance condition with the output voltage of the charge pump circuit applied to the output terminal connected to the substrate of the semiconductor integrated circuit for the video signal, it is feared that the semiconductor integrated circuit causes latch-up. In order to prevent the latch-up, the conventional semiconductor integrated circuit short-circuits between the output terminal and a GND potential during off of the charge pump circuit.
In addition, in JP-A-5-29360, a booster to prevent generation of latch-up when a control signal to be input in a charge pump circuit switches from High to Low is disclosed.
According to the above-described conventional general semiconductor integrated circuits, the video signal processing circuit block and the audio signal processing circuit block are mounted on the substrates of the separate semiconductor integrated circuits, respectively. Thus, if the both semiconductor integrated circuits are mounted on the product, there is a problem that the area on which the semiconductor integrated circuit is mounted is very large on the substrate of the set product. Therefore, it is preferable that the video signal processing circuit block and the audio signal processing circuit block are mounted on one substrate. However, in the case of mounting the conventional video signal processing circuit block and audio signal processing circuit block on one substrate, the following problem may occur.
The conventional semiconductor integrated circuit short-circuits between the output terminal and the GND potential in order to prevent latch-up. If the output terminal and the GND potential are short-circuited, the voltage of the output terminal is rapidly changed. As the conventional case, when the video signal processing circuit block and the audio signal processing circuit block are mounted on the separate semiconductor integrated circuits, a parasite capacity is not formed between the semiconductor integrated circuits for the video and audio signal. Therefore, even if the voltage of the output terminal of the semiconductor integrated circuit for the video signal is rapidly changed, variance of the voltage of the output terminal is not transferred to a P-type substrate of the semiconductor integrated circuit for the audio signal. However, in the case of mounting the video signal processing circuit block and the audio signal processing circuit block on one substrate, the variance of the voltage of the output terminal is transferred to the audio signal processing circuit block, thus resulting in a problem such as a transient sound or the like occurs in the audio signal processing circuit block.
In addition, in the case of switching a state that the audio signal processing circuit block operates only into a state that the video signal processing circuit block operates again, that is, in the case of switching the charge pump circuit from off into on to isolate the output terminal of the charge pump circuit from the GND potential, the voltage of the output terminal of the semiconductor integrated circuit for the video signal is varied from the GND potential to the output voltage of the charge pump circuit. The declination of this voltage variance is determined by an ability (hereinafter, referred to as “an output impedance of a charge pump circuit”) to discharge the voltage from the charge pump circuit to a capacitor for holding the voltage connected to the output terminal. Mounting the video signal processing circuit block and the audio signal processing circuit block on one substrate, when the declination of the voltage variance is steep, there is also a problem that the operation of the audio signal processing circuit block in the operating state is influenced.